During the combustion process, gasoline is oxidized and hydrogen (H) and carbon (C) combine with air. Various chemical compounds are formed including carbon dioxide (CO2), water (H2O), carbon monoxide (CO), nitrogen oxides (NOx), unburned hydrocarbons (HC), sulfur oxides (SOx), and other compounds.
Automobile exhaust systems include a catalytic converter that reduces the levels of CO, HC, and NOx in the exhaust gas by chemically converting these gasses into carbon dioxide, nitrogen, and water. Diagnostic regulations require periodic monitoring of the catalytic converter for proper conversion capability. Typical monitoring methods employ two exhaust gas oxygen sensors and infer the conversion capability of the catalytic converter using the sensor signals. One sensor monitors the oxygen level associated an inlet exhaust stream of the catalytic converter. This inlet O2 sensor is also the primary feedback mechanism that maintains the fuel-to-air (F/A) ratio of the engine at the chemically correct, or stoichiometric F/A ratio needed to support the catalytic conversion processes. A second or outlet O2 sensor monitors the oxygen level concentration of the exhaust stream exiting the catalytic converter. Excess O2 concentration in the exiting exhaust stream induces a “lean” sensor signal. A deficit or absence of O2 in the exiting exhaust stream induces a “rich” sensor signal.
Exhaust stream O2 sensors are categorized as either narrow range or wide range. The terms narrow and wide refer to the size of the F/A window that the O2 sensor varies in an analog fashion. Narrow range exhaust stream O2 sensors are sometimes referred to as “switching” sensors. These sensors transition between lean and rich sensor signals in a narrow F/A ratio range that brackets the stoichiometric F/A ratio. Wide range exhaust steam O2 sensors widen the analog transition range into the lean F/A ratio range to control engines having stratified charge or lean burn combustion.
Traditional monitoring methods relate the empirical relationships that exist between the inlet and outlet O2 sensor to quantify catalyst conversion capability. These methods compare sensor amplitude, response time, response rate, and/or frequency content data. All of these measurements are affected by a property of a catalytic converter known as Oxygen Storage Capacity (OSC). OSC refers to the ability of a catalytic converter to store excess oxygen under lean conditions and to release oxygen under rich conditions. The amount of oxygen storage and release decreases as the conversion capability of the catalytic converter is reduced. Therefore, the loss in OSC is related to the loss in conversion capability.